这个错误是由autoresizingmask自动创建的约束引起的，它们的创建是因为UIView属性translatesAutoresizingMaskIntoConstraints默认为true。

考虑使用BoxView来摆脱所有手工约束创建样板，并使您的代码简洁易读。用BoxView做布局是非常简单的:

boxView.items = [
   new_view.boxed.centerX().centerY().relativeWidth(1.0).relativeHeight(1.0)
]

在xcode 7.3.1中略有不同。这是我想到的

   // creating the view
        let newView = UIView()
        newView.backgroundColor = UIColor.redColor()
        newView.translatesAutoresizingMaskIntoConstraints = false
        view.addSubview(newView)

        // creating the constraint 

        // attribute and relation cannot be set directyl you need to create a cariable of them
        let layout11 = NSLayoutAttribute.CenterX
        let layout21 = NSLayoutRelation.Equal
        let layout31 = NSLayoutAttribute.CenterY
        let layout41 = NSLayoutAttribute.Width
        let layout51 = NSLayoutAttribute.Height
        let layout61 = NSLayoutAttribute.NotAnAttribute

        // defining all the constraint
        let horizontalConstraint = NSLayoutConstraint(item: newView, attribute: layout11, relatedBy: layout21, toItem: view, attribute: layout11, multiplier: 1, constant: 0)
        let verticalConstraint = NSLayoutConstraint(item: newView, attribute: layout31, relatedBy: layout21, toItem: view, attribute: layout31, multiplier: 1, constant: 0)
        let widthConstraint = NSLayoutConstraint(item: newView, attribute: layout41, relatedBy: layout21, toItem: nil, attribute: layout61, multiplier: 1, constant: 100)
        let heightConstraint = NSLayoutConstraint(item: newView, attribute: layout51, relatedBy: layout21, toItem: nil, attribute: layout61, multiplier: 1, constant: 100)

        // adding all the constraint
        NSLayoutConstraint.activateConstraints([horizontalConstraint,verticalConstraint,widthConstraint,heightConstraint])

尝试这个优雅的UIView扩展约束。你可以简单地做约束:

 - firstView.coverWholeSuperview()
 - firstView.constraints(size: CGSize(width: 44, height: 44), centerX: view.centerXAnchor, centerY: view.centerXAnchor)
 - firstView.constraints(top: view.topAnchor, 
                         leading: secondView.leadingAnchor, 
                         bottom: view.bottomAnchor, 
                         trailing: secondView.trailingAnchor, 
                         padding: UIEdgeInsets(top: 12, left: 12, bottom: 12, right: 12))

这里是扩展，只需复制到您的项目。

extension UIView {
    /// Attaches all sides of the receiver to its parent view
    func coverWholeSuperview(margin: CGFloat = 0.0) {
        let view = superview
        layoutAttachTop(to: view, margin: margin)
        layoutAttachBottom(to: view, margin: margin)
        layoutAttachLeading(to: view, margin: margin)
        layoutAttachTrailing(to: view, margin: margin)

    }

    /// Attaches the top of the current view to the given view's top if it's a superview of the current view
    /// or to it's bottom if it's not (assuming this is then a sibling view).
    @discardableResult
    func layoutAttachTop(to: UIView? = nil, margin: CGFloat = 0.0) -> NSLayoutConstraint {

        let view: UIView? = to ?? superview
        let isSuperview = view == superview
        let constraint = NSLayoutConstraint(item: self, attribute: .top, relatedBy: .equal,
                                            toItem: view, attribute: isSuperview ? .top : .bottom, multiplier: 1.0,
                                            constant: margin)
        superview?.addConstraint(constraint)

        return constraint
    }

    /// Attaches the bottom of the current view to the given view
    @discardableResult
    func layoutAttachBottom(to: UIView? = nil, margin: CGFloat = 0.0, priority: UILayoutPriority? = nil) -> NSLayoutConstraint {

        let view: UIView? = to ?? superview
        let isSuperview = (view == superview) || false
        let constraint = NSLayoutConstraint(item: self, attribute: .bottom, relatedBy: .equal,
                                            toItem: view, attribute: isSuperview ? .bottom : .top, multiplier: 1.0,
                                            constant: -margin)
        if let priority = priority {
            constraint.priority = priority
        }
        superview?.addConstraint(constraint)

        return constraint
    }

    /// Attaches the leading edge of the current view to the given view
    @discardableResult
    func layoutAttachLeading(to: UIView? = nil, margin: CGFloat = 0.0) -> NSLayoutConstraint {

        let view: UIView? = to ?? superview
        let isSuperview = (view == superview) || false
        let constraint = NSLayoutConstraint(item: self, attribute: .leading, relatedBy: .equal,
                                            toItem: view, attribute: isSuperview ? .leading : .trailing, multiplier: 1.0,
                                            constant: margin)
        superview?.addConstraint(constraint)

        return constraint
    }

    /// Attaches the trailing edge of the current view to the given view
    @discardableResult
    func layoutAttachTrailing(to: UIView? = nil, margin: CGFloat = 0.0, priority: UILayoutPriority? = nil) -> NSLayoutConstraint {

        let view: UIView? = to ?? superview
        let isSuperview = (view == superview) || false
        let constraint = NSLayoutConstraint(item: self, attribute: .trailing, relatedBy: .equal,
                                            toItem: view, attribute: isSuperview ? .trailing : .leading, multiplier: 1.0,
                                            constant: -margin)
        if let priority = priority {
            constraint.priority = priority
        }
        superview?.addConstraint(constraint)

        return constraint
    }

    // For anchoring View
    struct AnchoredConstraints {
        var top, leading, bottom, trailing, width, height, centerX, centerY: NSLayoutConstraint?
    }

    @discardableResult
    func constraints(top: NSLayoutYAxisAnchor? = nil, leading: NSLayoutXAxisAnchor? = nil, bottom: NSLayoutYAxisAnchor? = nil,
                trailing: NSLayoutXAxisAnchor? = nil, padding: UIEdgeInsets = .zero, size: CGSize = .zero,
                centerX: NSLayoutXAxisAnchor? = nil, centerY: NSLayoutYAxisAnchor? = nil,
                centerXOffset: CGFloat = 0, centerYOffset: CGFloat = 0) -> AnchoredConstraints {

        translatesAutoresizingMaskIntoConstraints = false
        var anchoredConstraints = AnchoredConstraints()

        if let top = top {
            anchoredConstraints.top = topAnchor.constraint(equalTo: top, constant: padding.top)
        }

        if let leading = leading {
            anchoredConstraints.leading = leadingAnchor.constraint(equalTo: leading, constant: padding.left)
        }

        if let bottom = bottom {
            anchoredConstraints.bottom = bottomAnchor.constraint(equalTo: bottom, constant: -padding.bottom)
        }

        if let trailing = trailing {
            anchoredConstraints.trailing = trailingAnchor.constraint(equalTo: trailing, constant: -padding.right)
        }

        if size.width != 0 {
            anchoredConstraints.width = widthAnchor.constraint(equalToConstant: size.width)
        }

        if size.height != 0 {
            anchoredConstraints.height = heightAnchor.constraint(equalToConstant: size.height)
        }

        if let centerX = centerX {
            anchoredConstraints.centerX = centerXAnchor.constraint(equalTo: centerX, constant: centerXOffset)
        }

        if let centerY = centerY {
            anchoredConstraints.centerY = centerYAnchor.constraint(equalTo: centerY, constant: centerYOffset)
        }

        [anchoredConstraints.top, anchoredConstraints.leading, anchoredConstraints.bottom,
         anchoredConstraints.trailing, anchoredConstraints.width,
         anchoredConstraints.height, anchoredConstraints.centerX,
         anchoredConstraints.centerY].forEach { $0?.isActive = true }

        return anchoredConstraints
    }
}

这是一种以编程方式添加约束的方法

override func viewDidLoad() {
            super.viewDidLoad()


let myLabel = UILabel()
        myLabel.labelFrameUpdate(label: myLabel, text: "Welcome User", font: UIFont(name: "times new roman", size: 40)!, textColor: UIColor.red, textAlignment: .center, numberOfLines: 0, borderWidth: 2.0, BorderColor: UIColor.red.cgColor)
        self.view.addSubview(myLabel)


         let myLabelhorizontalConstraint = NSLayoutConstraint(item: myLabel, attribute: NSLayoutAttribute.centerX, relatedBy: NSLayoutRelation.equal, toItem: self.view, attribute: NSLayoutAttribute.centerX, multiplier: 1, constant: 0)
        let myLabelverticalConstraint = NSLayoutConstraint(item: myLabel, attribute: NSLayoutAttribute.centerY, relatedBy: NSLayoutRelation.equal, toItem: self.view, attribute: NSLayoutAttribute.centerY, multiplier: 1, constant: 0)
        let mylabelLeading = NSLayoutConstraint(item: myLabel, attribute: NSLayoutAttribute.leading, relatedBy: NSLayoutRelation.equal, toItem: self.view, attribute: NSLayoutAttribute.leading, multiplier: 1, constant: 10)
        let mylabelTrailing = NSLayoutConstraint(item: myLabel, attribute: NSLayoutAttribute.trailing, relatedBy: NSLayoutRelation.equal, toItem: self.view, attribute: NSLayoutAttribute.trailing, multiplier: 1, constant: -10)
        let myLabelheightConstraint = NSLayoutConstraint(item: myLabel, attribute: NSLayoutAttribute.height, relatedBy: NSLayoutRelation.equal, toItem: nil, attribute: NSLayoutAttribute.notAnAttribute, multiplier: 1, constant: 50)
        NSLayoutConstraint.activate(\[myLabelhorizontalConstraint, myLabelverticalConstraint, myLabelheightConstraint,mylabelLeading,mylabelTrailing\])
}

extension UILabel
{
    func labelFrameUpdate(label:UILabel,text:String = "This is sample Label",font:UIFont = UIFont(name: "times new roman", size: 20)!,textColor:UIColor = UIColor.red,textAlignment:NSTextAlignment = .center,numberOfLines:Int = 0,borderWidth:CGFloat = 2.0,BorderColor:CGColor = UIColor.red.cgColor){
        label.translatesAutoresizingMaskIntoConstraints = false
        label.text = text
        label.font = font
        label.textColor = textColor
        label.textAlignment = textAlignment
        label.numberOfLines = numberOfLines
        label.layer.borderWidth = borderWidth
        label.layer.borderColor = UIColor.red.cgColor
    }
}

想在Imanou Petit的回答中添加一些理论概念，这样人们就可以了解自动布局是如何工作的。

要理解自动布局，请将您的视图视为橡胶的对象，最初是收缩的。

要在屏幕上放置一个对象，我们需要4件强制性的事情:

物体的X坐标(水平位置)。 物体的Y坐标(垂直位置) 对象的宽度 物体的高度。

X坐标:有多种方式给视图X坐标。

如前向约束，后向约束，水平中心 等。

2 Y坐标:给视图Y坐标有多种方式:

如顶部约束，底部约束，垂直中心等。

有两种方法来限制一个视图的宽度:

a.添加固定宽度约束(将此约束视为固定宽度的铁棒，并与橡胶对象水平挂钩，使橡胶对象不会收缩或膨胀)

b.不要添加任何宽度约束，而是在视图的前端和前端添加x坐标约束，这两个约束将通过从前端和前端拉动橡胶对象来扩展/收缩。

对象的高度:类似于宽度，有两种方法来限制视图的高度:

a.添加固定高度约束(将此约束视为固定高度的铁棒，并与橡胶物体垂直挂钩，使橡胶物体不会收缩或膨胀)

b.不要添加任何高度约束，但在视图的顶部和底部两端添加x坐标约束，这两个约束将从两端、顶部和底部拉伸/推动橡胶对象。

Swift 3更新

import UIKit

class ViewController: UIViewController {

let redView: UIView = {

    let view = UIView()
    view.translatesAutoresizingMaskIntoConstraints = false
    view.backgroundColor = .red
    return view
}()

override func viewDidLoad() {
    super.viewDidLoad()

    setupViews()
    setupAutoLayout()
}

func setupViews() {

    view.backgroundColor = .white
    view.addSubview(redView)
}

func setupAutoLayout() {

    // Available from iOS 9 commonly known as Anchoring System for AutoLayout...
    redView.leftAnchor.constraint(equalTo: view.leftAnchor, constant: 20).isActive = true
    redView.rightAnchor.constraint(equalTo: view.rightAnchor, constant: -20).isActive = true

    redView.centerYAnchor.constraint(equalTo: view.centerYAnchor).isActive = true
    redView.heightAnchor.constraint(equalToConstant: 300).isActive = true

    // You can also modified above last two lines as follows by commenting above & uncommenting below lines...
    // redView.topAnchor.constraint(equalTo: view.topAnchor, constant: 20).isActive = true
    // redView.bottomAnchor.constraint(equalTo: view.bottomAnchor).isActive = true
 }
}

约束的类型

 /*
// regular use
1.leftAnchor
2.rightAnchor
3.topAnchor
// intermediate use
4.widthAnchor
5.heightAnchor
6.bottomAnchor
7.centerXAnchor
8.centerYAnchor
// rare use
9.leadingAnchor
10.trailingAnchor
etc. (note: very project to project)
*/